RE: [PATCH v3] media: uvc: don't do DMA on stack

[David Laight <David.Laight@ACULAB.COM>]

From: Mauro Carvalho Chehab
> Sent: 21 June 2021 14:40
> 
> As warned by smatch:
> 	drivers/media/usb/uvc/uvc_v4l2.c:911 uvc_ioctl_g_input() error: doing dma on the stack (&i)
> 	drivers/media/usb/uvc/uvc_v4l2.c:943 uvc_ioctl_s_input() error: doing dma on the stack (&i)
> 
> those two functions call uvc_query_ctrl passing a pointer to
> a data at the DMA stack. those are used to send URBs via
> usb_control_msg(). Using DMA stack is not supported and should
> not work anymore on modern Linux versions.
> 
> So, use a kmalloc'ed buffer.
...
> +	buf = kmalloc(1, GFP_KERNEL);
> +	if (!buf)
> +		return -ENOMEM;
> +
>  	ret = uvc_query_ctrl(chain->dev, UVC_GET_CUR, chain->selector->id,
>  			     chain->dev->intfnum,  UVC_SU_INPUT_SELECT_CONTROL,
> -			     &i, 1);
> +			     buf, 1);

Thought...

Is kmalloc(1, GFP_KERNEL) guaranteed to return a pointer into
a cache line that will not be accessed by any other code?
(This is slightly weaker than requiring a cache-line aligned
pointer - but very similar.)

Without that guarantee you can't use the returned buffer for
read dma unless the memory accesses are coherent.

	David

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